The present invention relates to an integrated orthogonal fluxgate magnetic sensor and method of the fabrication thereof.
There is generally a need in many applications for a low-cost miniature magnetic field sensor to measure a weak magnetic field with certain precision. Fluxgates are the most popular, high sensitivity magnetic sensors built using an easily saturable ferromagnetic core. Fluxgate type magnetic sensors may be used for magnitudinal and directional measurement of DC or low-frequency AC magnetic fields. Typical applications are electronic compasses, current sensors, magnetic ink reading, detection of ferrous materials, and non-destructive testing [1, 2]. The main advantage of fluxgate sensors are their high sensitivity and very low offset. On the other hand, low magnetic field operation range and high perming are problems in current fluxgate sensors [3].
The working principle of fluxgate sensors based on the periodic saturation of ferromagnetic material with an AC excitation field and to detect the change in the flux passing through the core, which is proportional to the external magnetic field. Two kinds of configurations in fluxgates are generally known: a parallel fluxgate, having the excitation field parallel to the measured field, and an orthogonal fluxgate with the excitation field perpendicular to the measured field.
In order to simplify the manufacturing, only one ferromagnetic core combined with coils can be used as a fluxgate sensor. When only one ferromagnetic core is used, the orthogonal fluxgate configuration is preferable because of better signal treatment dynamic of the fluxgate sensor. The magnetic field to be measured is physically decoupled from the excitation field by placing the sensing coil to the orthogonal position with respect to the excitation field. The contribution of the excitation field is then removed from the measured signal [6].
FIG. 1 illustrates an orthogonal configuration of a conventional single-core fluxgate sensor. The circular excitation magnetic field Hexc is orthogonal to the axis of the core (C) and consequently to the external applied magnetic field Hext. Such an arrangement decouples the sensing coil (S) from the excitation coil (E).
The conventional orthogonal fluxgate sensor shown in FIG. 1 is however costly to manufacture, particularly in view of the need to wind the excitation coil around the ferromagnetic core, a portion of the coil being fed through the central cavity provided in the core. Winding of the sensing coil around the core is also not particularly cost effective. The excitation coil winding also limits the practical length thereof. The construction also limits the degree of miniaturization and the possibilities of integration of the sensor in miniaturized electronic devices.
In view of the foregoing, several technologies have been developed in order to integrate the fluxgate sensor in a compact and cost effective arrangement. The planar configurations as described in [7], [8] or [9] use always an open-core structure in parallel configuration and differential mode, with the following characteristics:                two ferromagnetic cores or two parts of one ferromagnetic core,        magnetized in longitudinal direction of the core,        two cores magnetized in opposite directions—differential mode,        core dimensions (core length over core section area) and magnetic properties of the core determinate at once the sensor resolution and the requirements on excitation field performances,        the excitation of the core and/or the detection of the measured field is performed by the 3D micro machined coils enclosing the core, or planar coils situated under the core and fabricated by using the metallization in CMOS technology.        
Such known configurations of integrated fluxgate sensors, while being compact and cost effective to manufacture in large series, have the following drawbacks:                The use of two ferromagnetic cores relatively complicates the sensor configuration and occupies more space.        The open-core structure is not magnetized thoroughly along the whole length of the core. The core center saturates first, the tips of the core as the last. The tips of the core never achieve the deep saturation and it creates perming of the sensor (i.e. memory effect of the sensor to the hard magnetic shocks).        Limitation of the core dimensions for detection and excitation fields. The shorter the core, the wider the measuring range is at variance with the excitation condition, however the shorter the core, the more difficulty there is to generate a magnetic field with sufficient strength to saturate the core along the whole length. Therefore, the core length influences in opposite directions the measuring range and the excitation arrangement.        The use of planar coils for both excitation and detection leads to a closely coupled coil structure. Such a structure creates huge capacitive and magnetic parasitic signals compared to the measured signal. This disturbing signal deteriorates the sensor signal to noise ratio and stability.        